I am replacing a few interior doors and I need to know if the existing door is Right Handed or Left Handed so I have them drilled correctly. Sounds like the acronyms for this are RH and LH. This also would apply for new pre-hung doors as well I assume as they are all marked that way at the mega home improvement stores.

7 Answers
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Not only besides RH or LH, you need to determine swing direction. Trust me, this needs to be agreed upon. When I built the additional room onto my house, I thought 84 Lumber understood the direction of door swing and they didn't. There is left inswing, right inswing, left outswing, and right outswing.

The door handle on my exterior door is on the right-hand side, but it is considered a left-hand outswing. Whereas my front door swings in, and the handle is on the right hand side so it is considered a right-hand inswing. Make sure you know what you are buying before you spend the money!

This is true, but only applies to exterior doors, where it matters which side of the door faces outside. On interior doors it's enough to use @chris's rule of thumb on @Matt Dowell's answer.
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Mike PowellNov 12 '10 at 15:46

@Mike: Not true! Based on the door jamb, you must also apply the above rules for an interior door! It would make sense to just look at the existing door and see what type he needs to replace.
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staticxNov 12 '10 at 16:22

@0A0D: But in an interior door, there is no inswing or outswing, because there's no "in" or "out". Here's Lowe's page for prehung interior doors, there's no mention of inswing or outswing options: lowes.com/…
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Mike PowellNov 12 '10 at 18:42

As a homeowner, I have installed many doors, and every time I do I have to re-learn how to "determine door swing". Just make sure you write it down before you order the door, it is easy to get confused again when you go to your local home improvement store. This website should help:

IF YOU ARE REPLACING AN EXISTING DOOR:
With the door open, stand with your
back against the hinge jamb. If your
left hand is nearer the doorknob, then
the door is LEFT-HANDED. If your
right hand is nearer the doorknob,
then the door is RIGHT-HANDED.

IF YOU ARE INSTALLING A NEW DOOR AND
JAMB IN A ROUGH OPENING: Decide
which side of the frame will have the
door hinges. Stand with your back
against the hinge-side of the frame
and extend an arm in the direction you
wish the door to open. If you
extended the left arm, then you need
to order a LEFT-HANDED door set. If
you extended the right arm, then you
need to order a RIGHT-HANDED door set.

IF THE DOOR IS GOING TO BE AN EXTERIOR
DOOR... Though exterior doors
typically swing inward, outswinging
exterior doors are available. So be
sure to add "inswinging" or
"outswinging" to your notes before
ordering!

whether a door is an exterior or iinterior door the designation is the same. Think of RH as right hinge. if the door is swinging away from you the hinge is on the right side. for a left hinge door, if the door is swinging away from you the hinges are on the left side. Inswing or outswing only matters with exterior doors where they have a sill. The handing or hinging is still determined the same way.

With the door closed, stand on the hinged side of the door. If you can see the hinges with the door closed you are on the hinged side of the door. If your left hand is closest to the door knob it is a left hand door. If your right hand is closest to the door knob it is a right hand door. This method works in the store where the doors are strapped shut. On exterior doors make sure some one didn't set the door down upside down; this will change things a bit.

Stand in such a way that the door will swing open towards you.
The handedness of the door is the hand that one would naturally use to grab the doorknob to pull the door open.

Inswing and outswing have to do with the way the jamb is constructed and the orientation of the lock bolt. If you're installing interior doors in an existing frame, and doing the lockset yourself, then there's no need to worry about in and out swings. If you're ordering pre-hung interior doors, again it doesn't matter, but make sure you're standing on the right side of the opening when you make your decision.

I modified 0A0D's image with some "L --- ^ --- R" labels . The V or ^ indicates which way you are facing.